1. Field of the Invention
The present invention relates to an ink composition and an inkjet recording method using the same.
2. Related Art
As an image recording method for forming an image on an inkjet recording medium such as paper with image data signal, there is an electrophotographic method, a sublimation type and a fusion type heat transfer method, an inkjet method and the like. For example, with respect to the inkjet method, it can be carried out by using an inexpensive device and an image is formed directly on a recording medium by jetting ink only on a required image part to achieve efficient use of ink, and therefore running cost is low. In addition, the inkjet method generates little noise and is an excellent image recording method.
According to the inkjet method, printing can be made not only on a regular paper but also on a recording medium which has no absorption property, for example, a plastic sheet, a metal plate and the like. In this case, however, increasing the printing speed and improving the image quality are an important subject to achieve. In addition, the inkjet method is characterized in that the time required for drying and curing of droplets after printing has a huge effect on productivity of a printing material or sharpness of a printing image.
As an image recording method using an inkjet method, a recording method using inkjet recording ink, which can be cured by radiation, can be mentioned. According to this recording method, radiation is applied right after ink ejection or after a certain period of time and then ink droplets are cured so that a sharp image with improved printing productivity can be formed.
By achieving high-sensitization of an inkjet ink composition which can be cured by radiation such as UV ray, high radiation curability can be obtained, and many advantages including improved productivity of inkjet recording, reduced power consumption, longer life time due to decreased load on radiation generator, and inhibited volatilization of a low molecular weight material based on insufficient curing. can be obtained. In addition, high-sensitization is particularly important for improving strength of a formed image.
An inkjet method based on curing by radiation, for example UV ray, recently draws attention because it generates relatively little odor, dries fast, and recording can be made even on a recording medium having no ink absorptivity. An ink composition for UV ray curing type inkjet in which radical polymerization is used has been suggested (for example, see, Japanese Patent Application Laid-Open (JP-A) Nos. 63-235382, 3-216379, 5-214280, Japanese Patent Application Publication (JP-B) Nos. 6-21256 and 6-62905).
In addition, as an inkjet ink composition which has high curing sensitivity and can provide an image with good quality, an inkjet ink composition which includes at least one hydrophobic polymer selected from the group consisting of a fluorine-containing polymer, a long chain alkyl group-containing polymer and an alicyclic group-containing polymer is known (for example, see JP-A No. 2006-182970).
However, such an inkjet ink composition is problematic in that a sticky image is produced or a low molecular weight component is exuded from image surface (so-called, “oozing out”), and thus blocking resistance is insufficient.
Meanwhile, as for the ink composition for a UV ray curing type inkjet, benzyl, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, 2-ethyl anthraquinone and the like have been generally used as an initiator for photopolymerization (for example, see, Bruce M. Monroe, et. al., Chemical Reviews, Vol. 93 (1993), pp. 435-448).
However, when such initiator for photopolymerization is used, because curing responsivity of an ink composition is poor, a long period of time is required for image exposure for forming an image. Thus, when a fine image is to be produced, an image with good quality cannot be reproduced if there is a subtle vibration during operation. In addition, since increased amount of energy radiation is required for exposure light source, liberation of a great amount of heat generated therefrom has to be taken into consideration.
In general, with respect to a radiation curing type ink composition, use of various kinds of polymerization initiation system is described as a way of improving sensitivity to radiation (see, Bruce M. Monroe, et. al., Chemical Reviews, VOl. 93 (1993), pp. 435-448, U.S. Pat. No. 4,134,813, JP-A Nos. 1-253731 and 6-308727). However, there is no example in which a polymerization initiation system, which has satisfying sensitivity to scanning exposure, is adopted for an ink composition.
In addition, jetting stability is also required for an inkjet ink composition.
As it has been discussed above, development of an ink composition, which can be cured by radiation with high sensitivity and form an image with high jetting stability in an inkjet recording, has been expected.